I have a scene in my story where the main character (the story is told from his POV) opens a door and there is a monster in the room.
I want a sudden introduction, to show how shocked the character is, but I also want to give a description of the monster, or at least give an idea of what it looks like.
How can I achieve both in the same paragraph?
The character was not expecting the monster, because monsters should not exist in his world.
Considering that a paragraph is intended to convey a single self contained concept, trying to convey a description of the monster and his reaction is going to make it unwieldy.
Imagine your reaction to entering that room, you're not going to pause your terror while you make a complete mental note of what it is and what it looks like. You're going to be terrified at this thing coming at you (or whatever its doing) once that terror has subsided you would then start paying attention to what it is
So tell the reader that something scary happened, explain how he reacted to that, then explain a little more of what he is seeing, and explain his reaction to that...
Don't be too eager to spoil the suspense for the reader, let them wait to find out what's going on! Make them wonder for a while...
The first time the character sees the monster, he's only going to get a few basic details. Christ, that thing is huge! It's green! and the teeth!
After he's ducked out of the way and looked back over his shoulder, then he's going to notice the matted fur, the slitted yellow eyes, the cracked horns, the sulphurous breath, et cetera. So you describe it through the POV of the character and his terror.
Ditto MichaelB and LaurenIpsum.
Other alternative: Find a way to introduce the monster first in a bland context. Like have the characters discuss the legends about this monster, describe it, etc. Then when the characters sees it, you just have to say "There was a frambar in the room!" Depending on how it's done, this may telegraph to the reader that the characters will meet one of these later. That may be a good thing or a bad thing, depending on what you're trying to achieve. If you don't want to foreshadow it, you can give the description in a way that gives the impression that something ELSE about this scene is what's important and the mention of the monster is just a side note.
